Under the 3rd Generation Partnership Project (3GPP), LTE-Advanced LTE (LTE-A), an expansion of LTE, is being investigated. Under LTE-A, wireless carriers (carrier) of an LTE system are bundled affording a broader range of transmission band than the LTE system and thereby enabling higher data transmission capacity. The respective LTE wireless carriers that are bundled are referred to as component carriers.
An LTE compliant terminal transmits and receives data using one of the component carriers that a LTE-A base station has. An LTE-A compliant terminal (LTE-A terminal) uses one component carrier or simultaneously uses multiple component carriers to transmit and receive data. The number of component carriers and the center frequency of each component carrier are assumed to differ between adjacent base stations. Furthermore, continuous as well as discontinuous arrangement along the component carrier frequency axis at a given base station is also assumed.
A terminal in idle mode is registered (terminal position registration) in one tracking area. Tracking area information is unique information that differs for each terminal. Multiple base stations belong to one tracking area. The base stations that belong to each tracking area differ for each terminal.
When an idle terminal is paged, all of the base stations in the tracking area to which the terminal belongs transmit a paging signal. Upon receiving the paging signal, the terminal prepares to transition from the idle mode to an active mode. When the terminal, for example, transitions from the idle mode to the active mode, a network notifies the terminal of information (identifying information of a base station belonging to the tracking area) related to the tracking area of the terminal.
The idle terminal periodically receives a common control signal transmitted from each cell and based on the identifying information that is unique to a base station and included in the received common control signal, the terminal determines whether the terminal is still in the tracking area or outside the tracking area. When outside the tracking area, the terminal accesses a base station that has transmitted the common control signal or the base station that has the greatest reception power and is the nearest. The terminal further has the base station update the position registration information of the terminal. As a result, the tracking area to which the terminal belongs is updated.
None of the base stations can ascertain whether a given idle terminal is within the cell thereof. Although a mobility management entity (MME), which is an entity at a higher level than the base stations, can determine that the given idle terminal is among the base stations that are subordinate to the MME, the MME cannot ascertain in which base station cell the given idle terminal is located. When the idle terminal is paged, a paging signal is transmitted from the cells of all of the base stations belonging to the current tracking area of the terminal.
When an LTE-A terminal transitions from the active mode to the idle mode, the base station that managed the terminal in the active mode ceases management of the terminal. Having transitioned to the idle mode, the terminal receives a common control signal from one component carrier among multiple component carriers. Even if the terminal moves from the current cell to another cell, provided the terminal does not leave the tracking area, the other cell also does not recognize the presence of the terminal. When the terminal is paged from the network, base stations belonging the tracking area in which the terminal is currently present, transmit a paging signal. For reference, see, for example, 3GPP TS36.300, V8.8.0, March 2009 and 3GPP TS36.304, V8.5.0, March 2009.
However, the technology above poses a problem in that paging signals cannot be efficiently transmitted and received. For example, when the same paging signal is received from the component carriers of all of the base stations, the efficiency of wireless resource utilization diminishes. Further, for the base stations within the tracking area, whether an idle terminal is present within the cell thereof is unclear and by consequently transmitting the same paging signal, the efficiency of wireless resource utilization diminishes.
Further, if the paging signal is transmitted via a portion of the component carriers of the base stations, it is unclear whether a paging signal is transmitted via any of the component carriers and thus, the terminal cannot receive the paging signal. On the contrary, if the terminal receives the paging signal from all of the base station component carriers, power consumption of the terminal in the idle mode increases.
In light of the problems above, an object of one aspect of the present invention is to provide a communication system, a base station, a mobile terminal, and a communication method that solve the problems above and perform highly efficient transmission and reception of paging signals.